Micro electro mechanical system

ABSTRACT

Embodiments of a micro electro mechanical system are disclosed.

BACKGROUND

Micro-electro mechanical systems (MEMS) devices are a combination ofmicro mechanical and micro electronic systems.

Some MEMS devices may include two chips wired together. It can bedifficult to achieve the desired alignment between the two chips.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments of the presentsystem and method and are a part of the specification. The illustratedembodiments are merely examples of the present system and method and donot limit the scope of the disclosure.

FIG. 1 illustrates a top view of a two layer single chip MEMS deviceaccording to one exemplary embodiment.

FIG. 2 illustrates a cross sectional view of the MEMS device of FIG. 1taken along section 2-2 in FIG. 1.

FIG. 3 is a flowchart illustrating a method of forming a MEMS deviceaccording to one exemplary embodiment.

FIG. 4A is a side view of a MEMS device according to one exemplaryembodiment

FIG. 4B is a side view of a MEMS device according to one exemplaryembodiment.

FIG. 4C is a bottom view of a MEMS device, according to one exemplaryembodiment.

FIG. 5 is a side view of a MEMS device formed from a silicon oninsulator wafer, according to one exemplary embodiment.

Throughout the drawings, identical reference numbers designate similar,but possibly not identical, elements.

DETAILED DESCRIPTION

The present specification discloses an exemplary system and method forforming a micro-electro mechanical system (MEMS) transducer. Accordingto one exemplary embodiment disclosed herein, the MEMS transducer isformed from two wafers and decouples the thickness of the proof mass andflexures, thereby allowing each to be independently designed.Additionally, the present exemplary system and method etches both sidesof the wafer defining the flexures and the proof mass, allowing foroptical alignment of the top and bottom wafers. Further details of thepresent MEMS transducer system and method will be provided below.

Before particular embodiments of the present system and method aredisclosed and described, it is to be understood that the present systemand method are not limited to the particular process and materialsdisclosed herein as such may vary to some degree. It is also to beunderstood that the terminology used herein is used for the purpose ofdescribing particular embodiments and is not intended to be limiting, asthe scope of the present system and method will be defined by theappended claims and equivalents thereof.

As used in the present specification and in the appended claims, theterm “proof mass” is meant to be understood broadly as including anypredetermined inertial mass used in a measuring device or machine, suchas in acceleration measurement equipment, which serves as the referencemass for the quantity to be measured.

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present system and method for forming amicro-electro mechanical system (MEMS) transducer. It will be apparent,however, to one skilled in the art, that the present method may bepracticed without these specific details. Reference in the specificationto “one embodiment” or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment. The appearance of the phrase “inone embodiment” in various places in the specification may possiblyrefer to different embodiments.

Exemplary Structure

FIGS. 1 and 2 illustrate various views of a MEMS device (100) accordingto one exemplary embodiment. In particular, FIG. 1 illustrates a topview of the MEMS device (100) while FIG. 2 illustrates a cross sectionalview of the MEMS device taken along section 2-2 in FIG. 1. As shown inFIG. 2, the MEMS device generally includes an upper wafer (110)positioned above a lower wafer (120). A material (135) bonds the twowafers (110, 120) together to form a single chip. As will be describedin detail below, the illustrated two wafer configuration may provide forincreased processing speed and allows for a number of design andmanufacturing benefits.

FIG. 1 illustrates a top view of the upper wafer (110) in detail,according to one exemplary embodiment. As illustrated, the upper wafer(110) includes an anchoring portion (125) and a movable portion (130). Aplurality of coupling blocks (140) are located about the periphery ofthe upper wafer (110) such that the coupling blocks (140) are locatedbetween the anchoring portion (125) and the movable portion (130). Asshown in FIG. 1, gaps are formed between the coupling blocks (140) andthe movable portion (130) as well as between the coupling blocks (140)and the anchoring portion (125).

A plurality of first flexures (145) couple the movable portion (130) ofthe upper wafer (110) to the coupling blocks (140). Further, any othernumber of flexures such as the illustrated second flexures (150) cancouple the coupling blocks (140) to the anchoring portion (125). Theillustrated configuration allows the movable portion (130) of the upperwafer (110) to move parallel to both the x-axis and the y-axis.

Turning now to FIG. 2, a cross sectional view of the MEMS device takenalong section 2-2 in FIG. 1 is illustrated. As shown, a material (135)is illustrated as being significantly thicker than the wafers forpurposes of illustrating all the features of the MEMS device (100). Itwill be apparent that the thickness of the material (135) and the upperand lower wafers (110, 120) may have proportions other than those shownin FIG. 2. In one exemplary embodiment, the thickness of the material(135) between upper wafer (110) and the bottom wafer (120), which,according to one exemplary embodiment, is approximately equal to the gapbetween the wafers. According to one exemplary embodiment, the thicknessof the material (135) may be approximately 0.1 to 10 microns.Furthermore, the thickness of the upper and lower wafers (110, 120), maybe between about 300-725 microns. The fabrication of the MEMS transducermay also create a cavity (160) on the upper surface of the MEMS device(100). This cavity (160) may, according to one exemplary embodiment, beselectively defined during formation of the MEMS transducer. The cavity(160) may exist in a vacuum, according to one exemplary embodiment.

While the above-mentioned MEMS device (100) is described and illustratedas including two coupled wafers, any number of wafers may be used toform a MEMS device, according to the present exemplary system andmethod. According to one alternative embodiment, a third wafer, whichmay be manufactured from any number of materials including, but in noway limited to, glass, plastic, silicon, and the like, may be bonded tothe back surface of the MEMS device illustrated in FIGS. 1 and 2 to sealthe resulting structure. Alternatively, any number of protectiveunprocessed surfaces may be used to seal the MEMS device. The resultingMEMS device could then be sealed in a package.

Additionally, the use and formation of two or more wafers to form thepresent exemplary MEMS devices allows for the use of alternative waferstructures. Specifically, according to one exemplary embodiment, theupper wafer (410) may be formed from a silicon on insulator (SOI)substrate as illustrated in FIG. 5. Specifically, the ability to formthe upper wafer from an SOI substrate provides for greater control ofthe flexure and cavity formation. As illustrated in FIG. 5, an insulatorlayer (500) of the SOI substrate can be used as a guide in etching thesubstrate. Consequently, the thickness of the flexures (440) and theflexure edge can be readily established by the location of the insulatorlayer (500). Particularly, according to one exemplary embodiment, theheight of the resulting flexures (440) is defined by the height of thetop silicon on the SOI substrate and height of the proof mass (430) isdefined by the height of top silicon and height of the processed handlewafer.

Additionally, as illustrated in FIG. 5, etching the back side of theupper wafer (410) may create a cavity (510) above the proof mass (430).The ability to control the size of the back cavity allows for a designthat increases the mass of the proof mass (430) and the volume of theoverall MEMS package, which can be used to design for the pressurewithin a MEMS package. Particularly, according to one exemplaryembodiment, the x-y area of the cavity (510) can be extended to bebigger than x-y area of the proof mass (430) which results in highervolume under the proof mass (430) for a given cavity depth and henceassist in controlling the damping of the proof mass. Additionally,according to one exemplary embodiment, the ability to control the backcavity allows for an increased surface area and volume for a getter byconnecting the cavity to the external package volume, which can havemore surface area, through a deep trench. One exemplary method offorming a MEMS transducer device will now be discussed in further detailbelow.

Exemplary Formation

FIG. 3 is a flowchart illustrating a method of forming a MEMS transducerdevice, according to one exemplary embodiment. As illustrated in FIG. 3,the method begins by forming a first wafer (step 300). According to oneexemplary embodiment, formation of the first wafer may includepatterning a first side of the first wafer to form features therein.Patterning the wafer can include, but is by no means limited to,photolithography, imprint lithography, laser ablation, laser annealing,microcontact printing, inkjet printing, trenching, micromachining andthe like.

According to one exemplary embodiment, the features formed by theabove-mentioned processes may include the formation of any number ofcomponents including, but in no way limited to, flexures and/or one ormore movable portion. The relative size of each movable portioncorresponds with a proof mass of that movable portion. As used herein,and in the appended claims, the term “proof mass” shall be interpretedbroadly to include any predetermined inertial mass in a measuring deviceor machine, such as in acceleration measurement equipment, which servesas the reference mass for the quantity to be measured. The proof mass ofeach movable portion may be independently varied by adjusting thethickness and surface area of the proof mass. Varying the thickness canbe accomplished by grinding and/or etching the wafer using any of thesuitable presently available or future developed methods. Adjusting thesurface area can also include, but is not limited to patterning,etching, trenching, or micromachining.

Varying the thickness of the proof mass can be important to properfunctionality. For example, it may be desirable to form separate movableportions to separately detect movement relative to the x-y plane andparallel to the z axis. According to such a method, each movable portionmay have a predetermined mass. The mass of each movable portion may bedifferent, such that sensitivity to movement in the x-y and the z planesmay be independently selected, for a given die area, as desired withinthe same MEMS transducer device, thus selectively enhancing thesensitivity.

According to the present exemplary embodiment, the formation of the MEMStransducer device using multiple wafers, allows both sides of the wafersto be etched or otherwise modified. For example, according to oneexemplary embodiment, the formation of the flexures in the first waferinclude defining the flexure by removing material from a first side ofthe wafer and further removing material from a second side of the waferto release the flexure. According to one embodiment, the removal ofmaterial from the second side may be performed after multiple wafers arebonded, as will be described in further detail below.

The formation of the first wafer further includes the formation of oneor more electrically active plates on each of the movable portions. Thismay be performed using any number of deposition and/or patterningprocesses including, but not limited to, a vacuum deposition processes,a spin coating processes, a curtain coating processes, an inkjet coatingprocesses, and the like. For example, according to one exemplaryembodiment, a second side of the first wafer, which is located oppositeof the first side discussed above, may have electrical circuits and/orcomponents formed thereon. These electrically active plates may serve aselectrode plates or other electrical components as will be described inmore detail below. Further, formation of the movable portion may includethe formation of circuitry formed therewith and coupled to the electrodeplates and configured to be coupled to other components of the resultingMEMS transducer device. Electrically active plates may be patternedusing, but not limited to, any of the previously mentioned methods. Theformation of the circuitry can be done using any number of suitablepresently available or future developed methods.

As mentioned above, the present exemplary method also includes theformation of a second wafer (step 310). According to one exemplarymethod, formation of the second wafer includes the formation of a waferincluding, but not limited to, electrical components, such as electrodesand/or circuitry. According to one exemplary embodiment, the circuitryformed on the second wafer is configured to be electrically coupled tocircuitry in the first wafer through vias (490) and the like. Further,in order to electrically couple the first and second wafers, the presentexemplary method includes alignment and coupling of the first and secondwafers (step 320). Traditionally, aligning one or more wafers with anidentified target generally involves the use of a sophisticated machine,such as an aligner. The alignment process is done such that theelectrode plates in the first and second wafer are aligned relative toeach other. Before the alignment occurs, a bonding material may bedeposited on the surface of the first wafer. This bonding material couldbe, but is not limited to, an adhesive, or dielectric which provides ahermetic seal to the MEMS device. As used herein, and in the appendedclaims, the term “adhesive” shall be interpreted broadly as includinganything that may be used to join a plurality of substrates including,but in no way limited to, glue, solder, chemical bonding, plasmabonding, eutectic bonding and the like. Once the wafers have beenaligned and coupled, the electrode plates on each of the wafers areinitially aligned relative to one another. Relative movement of themovable portions of the upper wafer may then be monitored to detectmovement in the x-y plane and or parallel to the z-axis, as will now bediscussed in more detail.

While the presently formed first and second substrates may bemechanically aligned with an aligner, the present exemplary formationmethod also provides for optical alignment of the first and secondwafers. Specifically, according to one exemplary embodiment, the removalof material from both a first and a second side of the first waferallows for an optically viewable path through the first wafer.Consequently, physical alignment of the first relative wafer may beoptically checked during formation. Once aligned and physically coupledas mentioned above, the MEMS transducer may be used for measurement.

FIGS. 4A-4B illustrate a cross-section of an exemplary MEMS transducerdevice (400), according to an embodiment of the present exemplary systemand method, which incorporates many of the features of the MEMS device(100) shown in FIGS. 1 and 2. Specifically, FIGS. 4A-4B illustrate thecircuitry of the MEMS transducer device (400) in further detail.Referring to FIG. 4A, the MEMS transducer device includes an upper wafer(410) and a lower wafer (420). As mentioned above, the MEMS transducerdevice (400) is configured to detect movement using capacitor plates, orelectrodes, to detect movement of a moveable portion (430) of the upperwafer (410). The flexures (440) allow the moveable portion (430) to movein one or more of the x, y, or z directions in response to an externalforce, depending on the design of the system. For simplicity and ease ofillustration, the flexures (440) are shown schematically andgenerically. The upper wafer (410) is positioned above the lower wafer(420) and is connected thereto with an adhesive material (450).According to the exemplary illustrated embodiment, the wafers (410, 420)are bonded and sealed to form a single chip.

The present exemplary MEMS transducer device (400) illustrated in FIGS.4A-4B includes an electrode (460) on a lower surface of the movableportion (430) of the upper wafer (410). Electrodes (465) and (470) arelocated opposite electrodes on an upper surface of the lower wafer(420). As the movable portion (430) is agitated, the overlap between theelectrode (460) on the upper wafer (410) and the electrodes (465) and(470) on the lower wafer (420) varies causing a change in capacitancebetween the electrodes (460, 465, and 470). Movement of the MEMStransducer device (400) in the x and/or y direction is subsequentlydetected by detecting the change in capacitance. Detection is providedto the embedded circuitry by the at least one via

According to on exemplary embodiment, Equation 1 may be used tocalculate a change in capacitance between electrodes, where ε is thedielectric constant:

C=(εA)/d   Equation (1)

In Equation 1, A is the area of overlap between electrodes in the x andy direction and d is the distance between electrodes in the z direction.Use of equation 1 to calculate a change in capacitance betweenelectrodes is described in U.S. Pat. No. 6,504,385, entitled,“Three-Axis Motion Detector” by Hartwell et al, which is herebyincorporated by reference in its entirety.

Movement in the z direction may also be determined using another set ofelectrodes shown in FIG. 4B. According to one exemplary embodiment,these electrodes are coupled to a second movable portion of the MEMStransducer device. The electrode (475) located on the movable portion(430) and the electrode (480) located on the lower wafer (420), shown inFIG. 4B, are provided for determining movement in the z-direction.According to one exemplary embodiment, the upper electrodes (475)located on the moveable portion (430) may have a short length while theelectrode (480) located on the lower wafer (420) may extend the lengthof the moveable portion (430) such that the overlap between theelectrodes (475, 480) does not change. Consequently, any change incapacitance detected between the electrodes (475) and (480) issubstantially the result of movement in the z-direction.

Particularly, according to one exemplary embodiment illustrated in FIG.4C, the upper electrode (475) may include a plurality of electrodes(475A, 475B). According to one exemplary embodiment, the z-axisaccelerometer of the present exemplary system and method evaluates thedifferential distance between the two sides (475A, 475B) of the upperelectrode (475) relative to the lower electrode (480).

Further, returning again to FIG. 4B, the thickness of the movableportion (430) associated with detection of movement in the z-directionmay be formed as desired, substantially independent of the thickness ofthe movable portion (430) associated with detection of movement in thex-y plane. Consequently the present two-wafer structure allows themoveable portion (430) to have a larger distribution of inertial mass inthe x, y, or z directions than conventional single-wafer capacitive MEMStransducers. Furthermore, the x-y sensitivity and z sensitivity of thedevice may be independently selected and controlled.

The transducer electronics (485) shown in FIGS. 4A-4B may include, butare in no way limited to, one or more circuits detecting the change incapacitance between corresponding electrodes (460, 465, 470, 475, and480). According to one exemplary embodiment illustrated in FIG. 4B, theelectrodes (460, 475) on the moveable portion (430) are connected to thetransducer electronics (485) using the vias (490). The signal from theelectrodes (460, 475) is passed through the vias (490) to the transducerelectronics (485). For example, according to one exemplary embodiment,electrical signals may be transmitted from a circuit (not shown) on theupper wafer (410) to a circuit (not shown) on the lower wafer (420)through vias (490) or vice versa. Moreover conductors (not shown), forexample, running along the flexures (440), may be used to connectcircuits on the movable portion (430) of the upper wafer (410) to thevias (490). In the various embodiments listed above, one or morecircuits and electrodes may be used in alternative embodiments dependingon the design of the MEMS device. Furthermore a circuit, as describedherein, may include, but is in no way limited to, passive components(e.g., capacitors, resistors, inductors, electrodes, etc.) and activecomponents (e.g., transistors, etc.), or a combination thereof. Theelectrodes (460, 465, 470, 475 and 480) are illustrated herein as beingprovided on surfaces of the upper and lower wafer (410 and 420).However, a circuit including active and/or passive components may beprovided on any of these surfaces. In addition, a circuit may includecomponents on more than one wafer.

The transducer electronics (485) are also connected to the electrodes(465, 470) on the lower wafer (420), according to one exemplaryembodiment. Consequently, the transducer electronics (485) are operableto detect the change in capacitance between the electrodes. Thetransducer electronics may comprise one or more circuits for calculatingthe change in overlap A and/or distance d between the electrodes, asused in Equation 1 above. Alternatively, the transducer electronics(485) may output the change in capacitance to an external circuit forcalculating the change in overlap A and/or distance d. According to oneexemplary embodiment mentioned above, the z-axis accelerometer of thepresent exemplary system and method evaluates the differential distancebetween the two sides (475A, 475B) of the electrode. Using Equation 1above, the distance d may be calculated from the change in capacitancebetween the electrodes (475, 480). Furthermore, if a value d has beendetermined, the overlap A may also be calculated from the change incapacitance detected between the electrodes (460, 465, 470) shown inFIG. 4A. While the present exemplary embodiment is directed to a MEMStransducer device, other two-wafer devices may also be used, as will nowbe discussed in more detail.

According to one exemplary embodiment, more electrodes may be used orthe size and shape of the electrodes may be varied for detecting changein capacitance in one or more of the x, y, and z directions. Accordingto one exemplary embodiment, five electrodes and five counter electrodesmay be used to detect movement in the x, y, and z directions. Also, alesser number of electrodes may be used if movement in one or twodirections is to be detected. Further, the present exemplary formationmethod may be used to form similar MEMS devices including, but in no waylimited to, gyroscopes, inertial sensors, rate sensors, and the like.

What has been described and illustrated herein are embodiments of thepresent exemplary systems and methods along with some of variations. Theterms, descriptions and figures used herein are set forth by way ofillustration and are not meant as limitations. Those skilled in the artwill recognize that many variations are possible within the spirit andscope of the present system and method, which is intended to be definedby the following claims—and their equivalents—in which all terms aremeant in their broadest reasonable sense unless otherwise indicated.

The preceding description has been presented to illustrate and describeexemplary embodiments. It is not intended to be exhaustive or to limitthe disclosure to any precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the disclosure be defined by the following claims.

1. A method of forming a micro-electro mechanical system (MEMS),comprising: forming a first wafer, including removing material from saidfirst wafer to define at least one flexure member and at least one proofmass; forming a second wafer including an electronic circuit; andbonding said first wafer to said second wafer, such that a gap isdefined between said first wafer and said second wafer; wherein athickness of said at least one flexure member is independent of athickness of said at least one proof mass.
 2. The method of claim 1,wherein forming said first wafer comprises: patterning a circuitrycomponent on a first side of said first wafer; removing material fromsaid first side of said wafer to define a first side of said at leastone flexure member; and removing material from a second side of saidfirst wafer to further define said at least one flexure member.
 3. Themethod of claim 2, wherein removing material from said first side ofsaid first wafer and said second side of said first wafer comprisesetching, trenching, or micromachining said wafer.
 4. The method of claim1, wherein forming said first wafer includes forming at least onecoupling block on said first wafer, wherein said coupling block isconfigured to couple at least a segment of said flexure to an anchoringportion of said first wafer.
 5. The method of claim 1, furthercomprising sealing said MEMS with a protective unprocessed surface orwafer.
 6. The method of claim 1, wherein bonding said first wafer tosaid second wafer comprises applying an adhesive between said firstwafer and said second wafer.
 7. The method of claim 1, wherein bondingsaid first wafer to said second wafer comprises optically aligning saidfirst wafer to said second wafer through a window formed in said firstwafer.
 8. The method of claim 1, wherein said removing material fromsaid first wafer to define at least one flexure member comprisesremoving a known amount of material to define a height and a width ofsaid flexure to generate a desired spring constant of said flexure. 9.The method of claim 8, wherein said first wafer is configured to allowsaid proof mass to translate parallel to said second wafer.
 10. Themethod of claim 8, wherein said first wafer is configured to allow saidproof mass to translate perpendicular relative to said second wafer. 11.The method of claim 1, wherein said first wafer comprises a silicon oninsulator (SOI) substrate.
 12. The method of claim 1, furthercomprising: defining a cavity with said first wafer, wherein said cavityis defined by subtracting a height of said proof mass from an originalthickness of said first wafer; and defining said cavity such that an x-yarea of said cavity is larger than an x-y area of said proof mass.
 13. Amicro-electro mechanical system (MEMS) accelerometer, comprising: afirst wafer; and a second wafer bonded to said first wafer; whereinmaterial is removed from at least a top surface and a bottom surface ofsaid first wafer defining at least one flexure member and at least oneproof mass; wherein distribution of a proof mass of an x-y accelerometerand a z accelerometer on said first wafer are independentlycontrollable.
 14. The MEMS accelerometer of claim 13, wherein said firstwafer defines a cavity on said top surface, wherein said cavity isdefined by subtracting a height of said proof mass from an originalthickness of said first wafer.
 15. The MEMS accelerometer of claim 14,further comprising connecting said cavity to an external package volumethrough a deep trench.
 16. The MEMS accelerometer of claim 13, whereinsaid first wafer comprises a window configured to allow for opticalalignment of said first wafer with said second wafer.
 17. The MEMSaccelerometer of claim 13, wherein said first wafer comprises a siliconon insulator (SOI) wafer.
 18. The MEMS accelerometer of claim 17,wherein: a height of said at least one flexure member is defined by aheight of a top silicon layer of said SOI wafer; and a height of saidproof mass is defined by a height said top silicon and a height of aprocessed handle wafer.
 19. A MEMS transducer device, comprising: afirst wafer; a second wafer, wherein material is removed from at least atop surface and a bottom surface of said second wafer defining at leastone flexure member and at least one proof mass, wherein said first waferis bonded to said second wafer; at least one circuit operable to detectmovement of said at least one flexure member; wherein a sensitivity ofsaid at least one circuit is independently controllable in an x-y planeand a z direction.
 20. The MEMS transducer of claim 19, furthercomprising: at least one via coupled to said first wafer; said at leastone via providing a path for electrical signals traveling through saidfirst wafer.